"I am endorsing Hillary, and all her lies and all her empty promises," O'Rourke continued. "It's the second-worst thing that can happen to this country, but she's way behind in second place. She's wrong about absolutely everything, but she's wrong within normal parameters."

While many Republicans are falling in line — however reluctantly — behind de facto GOP nominee Donald Trump, O'Rourke is joining other prominent conservative authors like Bill Kristol, George Will and Erick Erickson in continuing to oppose a Trump candidacy.

"This man just can't be president," O'Rourke said, alluding to the nuclear codes the commander-in-chief takes control of upon assuming office. "They've got this button — this briefcase. He's going to find it."